Microphone



y 1941- A. w. WILLIAMS Er'AL 2,242,964

MICROPHONE Filed Aug. 19, 1939 Arrow/e Patented May 20, 1941 MICROPHONE Alfred L. W. Williams, Cleveland, and John P. Arndt, Jr.,'East Cleveland, Ohio. asslgnors to 7 The Brush Development Company, Cleveland,

Ohio, a corporation of Ohio Application August 19, 1939, Serial No. 290,988

3 Claims.

This invention relates to acoustical devices and more particularly to microphones having uni-directional response characteristics.

It has been shown that by properly combining a microphone transmitter responsive to the pressure gradient of a sound wave and a microphone transmitter responsive .to the pressure of a sound wave, a uni-directional microphone is obtained. Uni-directional microphones of this type have been made in two difi'erent forms which are described, respectively, in United States Reissue Patent No. 19,115 granted March 13, 1934, to H. F. Olson et a1. and in United States Patent No. 2,126,437 granted August 9, 1938, to A. L. W. Williams.

Olson's uni-directional microphone comprises a ribbon type transmitter with the ribbon divided into two sections. One section is freely accessible on both sides to sound wave vibrations so that it is responsive to the pressure gradient of a sound wave and acts in conventional manner providing bi-directional response. The other section has only one side freely accessible to sound wave vibrations. The other side of said section is shielded from sound waves and consequently that section of the ribbon responds to the pressure component of a sound wave and provides non-directional response. In this type of microphone, in order to obtain the proper phase relations so that the voltages generated by the two sections will be additive for sounds reaching the front of the microphone and subtractive for sounds reaching the back of themicrophone, it is necessary to terminate the shielded side of the pressure responsive portion of the ribbon in an acoustical network the impedance of which is predominately resistive and large tions. Both types of transmitters have bi-directional. response characteristics. However,

there is a phase difierenceof 90 between the outputs of the pressure transmitter and either type of pressure gradient transmitter. Consequently, phase correcting networks are required so that the outputs'may be combined in additive relation for sounds reaching the front of the microphone and in subtractive relation for sounds reaching the back of the microphone. This type of microphone is small in size but this advantage is more or less ofiset by the necessity for employing phase-correcting networks.

The chief object of this invention is to provide an improved uni-directional microphone of the type comprising the combination of a'transmitter responsive to pressure gradient and a transmitter responsive to pressure, characterized by simple, compact construction and requiring no acoustical or electrical compensating networks.

Referring to the drawing Fig. 1 is a front elevation of a microphone embodying our invention shown with the protective housing in dotted outline.

Fig. 2 is a side elevation of the microphone of Fig. 1 shown with part of the protective housing broken away to reveal inner construction.

Fig. 3 is a top view of the pressure responsive transmitter of the microphone shown in Figs. 1 and 2.

Fig. 4 is a front view of the transmitter of Fig. 3, part being shown in section.

Fig. 5 is a schematic diagram of the microphone andits associated circuit.

Figs. 6 and '7 are schematic diagrams showing alternative connections for the transmitters of the microphone.

compared with the mechanical impedance of the ribbon. The acoustical resistance network comprises a long pipe containing sound absorbing material. The electrical connections of this type of microphone are simple but the acoustical system is bulky andtherefore undesirable ,be-

cause it increases the size of the microphone and tends to introduce phase errors at higher frequencies due to diffraction effects.

Williams uni-directional microphone employs a piezo-electric sound cell transmitter, or. a condenser type transmitter for the pressure responsive element. Such transmitters are small, compact units requiring no additionalacoustical networks; The pressure gradient portion of the microphone comprises two pressure responsive transmitter units such as piezo-electric sound cells or condensed transmitters placed side by side and connected together in electrical opposition; or alternatively, comprises one or more piezo-electric or condenser transmitters having both sides of the vibratory sound-sensitive member exposed to the sound wave vibragap defined by thepolepieces. The pole and magnet assembly is .held together in alignment. by the frame member 1, clamping plate '8 and screws 9, 9 and U. A supporting yokell is carried by microphonestand l2 and rotatably supports the microphone by clamping screws l3, I3 threaded into .projections la of frame member 1. p I p y Ribbon 3 is Iexposedon both sides to thesurrounding, fluid medium so that both sldesare freely, accessible to sound; wave vibrations.

two faces of the unit and consequently there is a resultant force which causes vibration of the ribbon. The voltage generated in the ribbon in consequence of its vibration is substantially in phase with the sound wave pressure for plane sound waves approaching one side and substantially 180 out of phase with the pressure for plane sound waves approaching the other side of the ribbon. The ribbon is connected by wires I4, I 4 to a transformer (not shown) housed below the transmitter in box 44. I

Transmitter 2 preferably is of the well known piezo-electric sound cell type. It comprises two sound cells I5, I5 disposed in parallel face to face relationship and spaced apart by spacing washers I6, I5. It is resiliently supported by thin yoke members l1, ll of rubber or the like which are supported by screws I8 and spacers I9 and 20 on insulating platform 2I which is supported above the pole pieces of transmitter I. The cell assembly is held together by screws 22, 22. For

a detailed description of the construction of sound cells I5 reference may be had to United States Patent 2,126,438 of A. L. Williams and 2,105,010 of C. B. Sawyer. Briefly they each comprise a hollow frame 23 in which are disposed in parallel face to face relationship a pair of flexing type piezo-electric units 24, 24. The inner faces of the piezo-electric units are shielded from the surrounding air by frame 23 and membranes 25, 25. Variations in pressure accompanying the sound wave approaching the cells from any direction act only on the outer face of each piezoelectric unit and consequently cause corresponding iiezure of the piezo-electric units with resultant generation of corresponding voltage at terminals 26, 25. The voltage issubstantially in phase with the pressure of the sound wave. Terminals 25 are connected to the microphone circuit by conductors 21, 21.

It will be noted that the microphone units I and 2 are symmetrically disposed about a common axis. Accordingly, for sound waves approaching the microphone from any direction in a plane which is perpendicular to the common axis .the outputs of the two transmitter units will be either substantially in phase or 180 outof phase depending on whether the waves approach the front or back of the microphone.

The voltage generated by the ribbonis much smaller than the voltage generated by the sound cells. Furthermore, the ribbon has a relatively low inductive impedance while the sound cells 1 have a relatively high capacitive impedance. Hence theoutputs of the two transmitters cannot be combined directly to obtain a uni-directional microphone. It is necessary to step up the output of the ribbon and then to combine it with the output of the sound, cells in a circuit that will not disturb the phase relationships between the outputs of the two transmitters. We have found that the circuit arrangement of Fig. 5 is very satisfactory for this purpose.

In Fig. 5, I is the ribbon microphone transmitter unit and 2 is the piezo-electric sound cell' transmitter unit. Transmitter I is connected to the primary winding 28 of a step-up transformer 29 which steps up the ribbon voltage to equal the crophone appears across it for all frequencies within the range to be received. It should be large compared with the impedance of transmitter 2 at the lowest frequency to be received and large compared with the impedance of the transformer and ribbon measured across secondary winding 30 at the highest frequency to be received. Transmitter 2 is usually the controlling element although with a very low output velocity transmitter I the turns ratio of transformer 29 might have to be made so large that the impedance measured across winding 30 would be very high at the higher frequencies. In most cases resistance 34 may be from 2 to 10 megohms. In order to avoid phase shift due to stray wiring capacity, especially if a high ratio transformer isnecessary to equalize the outputs of the transmitters, it is desirable to locate the amplifler tube 33 close to transmitter 2 and trans former 29. We have found, however, thatby observing certain precautions the amplifier may be located at a considerable distance from the microphone.

Fig. 6 illustrates the arrangement with the amplifier at a distance from the microphone. The circuit is similar to that of Fig. 5 except that a shielded conductor 35 is interposedbetween the microphone transmitters and the vacuum tube. In this arrangement the impedance of transmitter 2 and cable 35 in series act as a load on the transformer winding 30 at the higher frequencies since both impedances are predominately capacieffect of transmitter 2 and cable 35 does not seriously affect the stepped-up output of trans-' mitter I and is also low enough so that the cable 35 is the predominate part of the load on transmitter 2. Since the impedance of cable 35 and the impedance of transmitter 2 are both essentially capacitive, the loading effect of cable 35 ,does not aifect the phase of the voltage generated by the transmitter 2.

Furthermore, we have found that good performance may be obtained with the microphone and amplifier separated bya considerable distance by using a modified circuit such as shown in Fig. '7. In this case it is desirable to have a transmitter 2a made up of more than two high capacity transmitter units suchas piezo-electric sound cells I5, I5 connected in parallel to obtain :as large a capacity as is practical. Transmitter 2a is connected to 'a high inductance primary winding 36 of a step-down transformer 31. Transmitter I is connected to the primary 38 of a step-up transformer 39. The secondary 40 of tzansformer 31 and the secondary ll of transformer 39are connected inseries, the transformer We have found that satisfactory results ratios being so adjusted that the transformed outputs of transmitters I and. 2a appearing across secondary windings 4| and 40 respectively are equal for sound waves arriving at the front of the microphone normal to the ribbon. The impedance, looking back into windings 40 and 4| in series, should have a relatively low value so that the shunting impedance of relatively long conductors 42, 42 connected to the windings will not alter the outputs of the transmitters. Between the far ends of conductors 42 and amplifier 33 is interposed a transformer 43 to step the voltage up to as high a value as practical. By way of example, transformer 31 may be designed to match, over the audio range, an impedance of 100,000 ohms or higher to an impedance of 100 ohms. Transformer 43 may be designed to match, over the audio range, an impedance of 200 ohms to an impedance of 100,000 ohms or higher.

Transformer 39 may be designed to match ribbon impedance to a low impedance line, the actual -ratio being dictated by the voltage step-up necessary to equalize the outputs of transmitters l and 2a.

The combinations of transmitter units above described generally perform most satisfactorily when a single amplifier is used for both transmitters. If two amplifiers were used extra precautions would be necessary to prevent differences in the phase shift in the two amplifiers, as such differences would seriously reduce the unidirectional eflect of the combination. Further-' more, we have found that it is desirable to place the pressure transmitter 2 in a position where sound waves approaching it meet as little obstruction as possible since obstructions tend to cause phase errors with consequent reduction in the uni-directional efi'ect. With the conventional type of ribbon transmitter the best position is generally at the top directly over the ribbon as shown in Figs. 1 and 2.

Transformer 29 of Figs. 5 and 6 or transformers 31 and 39 of Fig. 7 may be housed in a box 44 carried below the microphone by frame member 7. For the arrangement of Fig. 5 the box 44 may be enlarged to house vacuum tube 33 or the tube 33 may be contained in a housing in the microphone stand.

Although it is desirable to use the type of transmitter units shown in the drawing, it is possible to obtain good results with other types. For example, the sound cells l5, I5 may be replaced by one or more diaphragm type piezo-electric transmitter units or by one or more condenser transmitter units, the chief requirements being that the transmitter be responsive to the pressure of the sound wave and have its output substantially in phase with the pressure. If the usual condenser microphone unit is used, it will be necessary to supply polarizing voltage in well known manner and to use a somewhat larger grid leak resistance at the input of the amplifier. It may-be impractical to utilize the circuit arrangement of Fig. 7 when condenser transmitter units are employed since they usually have an internal impedance which is much higher than that of piezo-electric transmitters and it is difficult to construct transformers with sufliciently high inductance to prevent loss of low frequency response when used with such high impedance transmitters.

It will be seen that by our invention we have combined in a peculiar manner a pressure transmitter of conventional design with a velocity transmitter of conventional design to obtain a microphone having uni-directional properties."

By mechanically combining these two transmitters in the peculiar mutual relation set forth we have provided a combination microphone having two outputs which are substantially in phase for sounds reaching the front of the combination and substantially out'of phase for sounds reaching the back. Consequently we have avoided the need for acoustical or electrical phase correction networks, and the outputs, except for relative amplitudes, are suitable for combining for uni-directional operating characteristics. The outputs are equalized by means of a transformer which constitutes part of a simple circuit by means of which the outputs are combined without disturbing the desirable phase relationship obtained by proper choice and placement of the transmitters.

What we claim is:

1. In directional microphone apparatus, the combination of a unidirectional microphone comprising an electro-magnetic-type velocity transmitter, a piezo-electric-type pressure transmitter closely associated with said velocity transmitter, and means for equalizing and combining in opposition the outputs of the two transmitters for sound waves approaching'the back of the microphone, and means comprising a trans former having its primary connected to the velocity transmitter; and a remotely located amplifier electrically connected to the uni-directional microphone.

2. In com-bination, a ribbon-type velocity transmitter having a maximum output for sound waves approaching the front and back thereof. a piezo-electric-type pressure transmitter closely associated with said velocity transmitter, means for equalizing the maximum output of the-velocity transmitter and the output of the pressure transmitter, said means including a step-up transformer having a primary winding connected to the velocity transmitter and a secondary winding connected in series with the pressure transmitter,

a remotely located amplifier, and a shielded transmitting line connecting the amplifier to the series com-bination of the pressure transmitter and the transformer secondary winding, the output of the velocity transmitter being sufficiently high in relation to the output of the pressure transmitter so that the loading effect of the series combina-' tion of the pressure transmitter and the shielded transmitting line does not seriously affect the stepped-up output of the velocity transmitter.

3. In combination, an electro-magnetic-type velocity transmitter, a piezo-electric type pressure transmitter closely associated with said velocity transmitter, means for combining the electrical outputs of said transmitters comprising a step-up transformer with its primary connected to said velocity transmitter and a step -down transformer with its primary connected to said pressure transmitter, the secondaries of said transformers being connected in series, and a remotely located amplifier having a low impedance input electrically connected to the series-connected secondaries of the said transformers.

ALFRED L. W. WILLIAMS. JOHN P. ARNDT, JR.

CERTIFICATE OF coRnEcrion. Patent No. 2,2h2,96h. Hay 2o, 191

ALFRED L.' W. WILLIAMS; ET AL.

It is hereby certified that error appears in the printed specifi cation of the above numbered patent requiring correction as follows: Page '1, first column, line 55, for the word "condensed" read --oondenser-; page 2, first column, line 6, for "different" read --difference--; page 5, second column, line 51, claiml, for "and" read --said-; andthat the said Letters Patent should be read with this correction therein that the same may conformto the record of the case in the Patent Office.

Signed and sealed this Zhth day of June, A. D. 19in.

Henry Van Arsdale, (.Seal) Acting Commissioner of Patents. 

